1. Field of the Invention
The present invention relates to a semiconductor device for driving a current load device that supplies current to a current-driven element such as an organic electroluminescent element, and a display device having the same. In particular, the present invention relates to a semiconductor device with a constant current circuit, and a display device.
2. Description of the Related Art
An organic EL (Electro-Luminescence) display device has organic EL elements that are self-emitting and have a fast light-emitting response, and has features such as thin, lightweight, a wide viewing angle, and excellent moving image display functionality. FIG. 1 is a block diagram illustrating the configuration of an organic EL display device. As illustrated in FIG. 1, with a passive matrix (PM) type organic EL display device, each pixel 101 in a display unit 100 comprises an organic EL element 110 and wiring such as a scanning line 112 and a data line 111, and with an active matrix (AM) type organic EL display device, each pixel 101 in a display unit 100 is formed with a pixel circuit 113 that supplies current to the organic EL element 110, in addition to the organic EL element 110 and wiring such as the scanning line 112 and the data line 111.
Such organic EL display device performs a horizontal scan that selects organic EL elements 110 or pixel circuits 113 on each line, according to the signal from a horizontal scan circuit 103. Then, for the period that is line-selected, appropriate voltage or current is supplied to each organic EL element 110 or each pixel circuit 113 on the selected line via each data line 111 from each output of the organic EL display device drive circuit. The current to flow to the organic EL element 110 is determined based on the supplied voltage or current, and the illumination brightness of the organic EL element 110 is adjusted and the image is displayed. Therefore, the illumination brightness of the organic EL element 110 is determined by the applied voltage value or the supplied current value to the organic EL element 110. Also, a linear relationship exists between the illumination brightness and the supplied current with regards to the organic EL element 110, and a non-linear relationship exists between the illuminating brightness and the applied voltage.
Conventional organic EL elements have the problem that elements deteriorate as the light emission time elapses, and the brightness corresponding to the applied voltage decreases as the light emission time elapses. However, since the time variation of brightness corresponding to the supplied current is smaller than the time variation of brightness corresponding to applied voltage, a drive method that supplies current to the organic EL element can maintain a higher display quality than a method that applies voltage to the organic EL element.
In order to suppress deterioration in display quality of the above-described AM-type organic EL display device, it is important that the current supplied from the driving transistor that is provided to the pixel circuit 133 and supplies current to the organic EL element 110 is according to the design thereof, even in the case that the current properties of the driving transistor in each pixel 101 differs from each other. FIG. 2 is a circuit diagram illustrating a voltage-write and current-drive type pixel circuit. The pixel circuit 133a of the voltage-write and current-drive type illustrated in FIG. 2 is supplied with voltage from an external drive circuit via a data line 111. In the case that properties of the driving transistor 114 in the pixel circuit 133a vary from one pixel to another, the current provided to the organic EL Element 110 also varies from one pixel to another, and the light emission brightness of the organic EL element 110 also varies from one pixel to another. In the event that the light emission brightness of the organic EL element 110 differs from one pixel to another, non-uniformity is generated in the display image, and therefore display quality deteriorates.
On the other hand, a pixel circuit of the current-write and current-drive type is supplied with current from an external drive circuit via a data line 111. FIG. 3 is a circuit diagram illustrating a current-write and current-drive type pixel circuit. With the pixel circuit 113b, in the state that short has occurred between the gate and drain of the driving transistor 114 by the control line 115, in other words, in the state that the switches 117 through 119 are on (continuity), the current supplied by the data line 111 is stored, and next, without the switches 117 through 119 on, the switch 120 conducts by the control line 116, and the stored current flows to the organic EL element. Thus, by providing a current copier circuit to the pixel circuit, current recording and current output can both be performed with one driving transistor, and therefore, changes of the supply current to the organic EL element due to the irregularities of the driving transistor properties can be reduced, and display quality can be improved.
A drive circuit for outputting the current capable of corresponding to the current-write and current-drive type pixel circuit 113b illustrated in FIG. 3 might be a drive circuit that current copier circuits are provided in a number according to the gradient (for example, reference K. Abe et al., “16-1: A Poli-Si TFT 6-bit Current Data Driver for Active Matrix Organic Light Emitting Diode Displays”, EURODISPLAY 2002 Proceeding, pp. 279-281). FIG. 4 is a block diagram illustrating the operation of a drive circuit described in K. Abe et al., “16-1: A Poli-Si TFT 6-bit Current Data Driver for Active Matrix organic Light Emitting Diode Displays”, EURODISPLAY 2002 Proceeding, pp. 279-281. As illustrated in FIG. 4, the drive circuit 128 provides the same number of current copier circuits as the type of reference current supplied from the reference current source 127. In other words, in the case wherein n (wherein n is a natural number) types of reference current is output from the reference current source 127, the drive circuit is provided with n number of current copier circuits. Also, these n numbers of current copier circuits are connected in parallel. The drive circuit 128 has a current recording state and a current output state, and during the current recording state, a reference current i is supplied to the output transistor 121 of the current copier circuit in the state that short has occurred between the gate and drain from the reference current source 127, and the gate voltage of the output transistor 121 at this time (the voltage corresponding to the reference current i) is recorded with the capacitor 129. On the other hand, during the current output state, short-circuiting between the gate and drain of the output transistor 121 is resolved, and by inputting voltage that corresponds to the reference current i of the output transistor 121 from the capacitor 129, the same size current as the reference current i can be output from the output transistor 121.
Therefore, with regard to the drive circuit 128, reference currents which are different each other are supplied to current copier circuits respectively, and the reference current is recorded in each current copier circuit. Then while placing the drive circuit 128 in a current output state, the presence or absence of current output from each current copier circuit can be determined by turning the switch element 130 provided to each current copier circuit on state or off (no continuity) state according to the display digital data input from an external unit. In this manner, by combining the current output from each current copier circuit within the drive circuit 128, the predetermined current can be output from the drive circuit. For example, in the case that three current copier circuits are provided to the drive circuit 128, and each current copier circuit is supplied with three types of reference current i0 through i2 that the current ratio each differs twofold, each current copier circuit will output three types of reference current i0 through i2 that the current ratio each differs twofold. Then, by combining the on or off state of the switch elements 130 provided to each current copier circuit, the output current i0 through i2 is combined, and including the case that the current is 0, eight types of current can be output. Now, the drive circuit 128 is provided for each data line 131 that is provided to the display unit, and the output current from each drive circuit 128 is supplied to the pixel circuit via the data line 131.
Further, Japanese Unexamined Patent Application Publication No. 2000-293245 proposes a constant current circuit supplying multiple reference currents that store appropriate current ratios, as a reference electric power supply source for outputting reference current to the drive circuit. FIG. 5 is a circuit diagram illustrating a constant current circuit described in Japanese Unexamined Patent Application Publication No. 2000-293245. As illustrated in FIG. 5, the constant current circuit has a circuit configuration that can generate multiple reference currents for a drive circuit for an organic EL display device, and comprises an operational amplifier 122 such as a CMOS operation amplifier, a V-I conversion unit 124 that is formed from a transistor Tr101 and a resistor 123 which the resistance value is Rc, and a current mirror circuit unit 125 that is formed from a mirror transistor Tr102 and current source transistors Tr103 through Tr105.
The V-I conversion unit 124 of this constant current circuit operates so as to output the current i (=Vin/Rc) found by dividing the voltage Vin input into the non-inversion input terminal of the operational amplifier 122 by the resistance value Rc of the resistor element 123, to the transistor Tr101, Tr102, and the resistor 123. At this time, the voltage between the gate and source of the transistors Tr102 through Tr105 in the current mirror circuit unit 125 are equal with each other, and therefore the three current source transistors Tr103 through Tr105 output a current determined by: the current capability ratio to the mirror transistor Tr102, and the current flowing to the mirror transistor Tr102. Therefore, for example, in the case that the channel length of three transistors Tr103 through Tr105 is made the same as the channel length of the mirror transistor Tr102, and the channel width is made equal to, double, and quadruple, respectively compared to the channel width of the mirror transistor Tr102, then the current i1 through i3 output from the current source transistor Tr103 through Tr105 will be equal to, double, and quadruple, respectively, of the current i (=Vin/Rc) that flows to the mirror transistor Tr2.
However, the above-described related art has problems, which will be described below. The output current in the constant current circuit described in Japanese Unexamined Patent Application Publication No. 2000-293245 is determined by the ratio of the current capability of the mirror transistor Tr102 and the current capability of the current source transistors Tr103 through Tr105, but even if the current capability ratio of each transistor is set by changing the channel width of the current source transistors Tr103 through Tr105, the current capability may not be according to design, due to the manufacturing process and so forth. In this case, because the current source transistor outputs a current that differs from the specified current ratio, a problem occurs wherein the accuracy of the output current of the drive circuit generated based on this output current decreases.
In particular, low temperature poly-crystal silicon thin film transistors (LTPS TFT) and amorphous silicon thin film transistors (a-Si TFT) and so forth have greater irregularities of current properties, and when a constant current circuit is formed using these transistors, the deterioration in accuracy becomes greater.
Therefore, a constant current circuit has been device that the output current ratio irregularities due to the transistor property irregularities can be adjusted, by providing multiple current mirror circuits, and adjusting the input voltage for each circuit. FIG. 6 is a circuit diagram illustrating a conventional constant current circuit that is capable of adjusting the output current ratio. The constant current circuit 126 has six circuit blocks I0 through I5 with differing output currents connected with each other in parallel. And the circuit block 10 has the source terminals of the P-type transistor Tr121_I0 and transistor Tr122_I0 connected to the source electrode VDD, and while gate terminals of transistor Tr121_I0 and transistor Tr122_I0 are connected to each other and connected to the drain terminal of the transistor Tr101_I0, and the first date terminal is connected to an external source, and the source terminal is connected to a ground electrode GND. Further, the drain terminal of the transistor Tr122_I0 becomes the output terminal. The circuit blocks I1 through I5 in the constant current circuit 126 the same as the circuit block I0, except in the cases that the channel width of the transistor is a width corresponding to an output current ratio, for example, two, four, eight, sixteen, or thirty-two times the channel width of the transistor provided to the circuit block I0.
The constant current circuit 126 has power source potential applied to the power source electrode VDD, and the negative power source potential is applied to the ground electrode GND, and at the same time, the voltage VR is input into the gate terminal of the transistor TR123 from an external power source. By doing so, a current i0 corresponding to the voltage VR0 is generated with the transistor Tr123 within the circuit block I0. This current i0 flows to the transistor Tr121 that is connected to the transistor Tr123. Further, since a size and a voltage between the gate and the source of a transistor Tr122 is equal to those of the transistor Tr121, the same current i0 flows also to the transistor Tr122. Thus, the current i0 is output from the circuit block I0. The operation of circuit blocks I1 through I5 are also the same as those that of circuit block I0, therefore in the case that the transistor properties have no irregularities, the current i0 through i5 can be output at the predetermined ratio by making the input voltage VR0 through VR5 equal, for example, i0:i1:i2:i3:i4:i5=1:2:4:8:16:32. However, in the event that the properties of the transistors Tr121, Tr122, and Tr123 are irregular, the current ratio as designed cannot be obtained, and therefore with the constant current circuit 126, the input voltage VR0 through VR5 is adjusted so that the current i0 through i5 becomes the designed value.
In general, the semiconductor device for driving a current load device of a display element such as an organic EL element have this type of constant current circuit provided for each of R, G, and B, and after adjusting the current ratio within the constant current circuits, the balance between the reference current output from the circuits and the RGB (white balance) is adjusted. In the constant current circuit 126 illustrated in FIG. 6, since the adjustment of this reference current and the white balance is performed by adjusting the input voltage VR0 through VR5, problems exist that the current ratio of currents i0 through i5 can easily differ from the designed value, and the reference current and the white balance become difficult to adjust while holding at this current ratio.